1. Field of the Invention
This invention relates to the regeneration of waste liquors containing nitric and hydrofluoric acids. More particularly, the invention relates to a process for continuously regenerating and recovering highly pure nitric acid and hydrofluoric acid from a waste liquor containing metal ions along with nitric acid and hydrofluoric acid as discharged from a pickling procedure wherein metal materials are pickled with nitric acid and hydrofluoric acid.
2. Description of the Prior Art
As is well known in the art, when stainless steels or titanium is annealed, metal oxides scales are inevitably formed on the surfaces of the steels or titanium. For the removal of the scales, usual practice is to pickle such stainless steels or titanium with a mixed acid mainly composed of nitric acid and hydrofluoric acid. As the pickling continues, Fe, Cr, Ni, Ti and the like metals are more and more dissolved in the mixed acid. This results in a reduced amount of the free acids present in the mixed acid, with a lowering of the pickling capability. Accordingly, substitution with a fresh mixed acid becomes necessary, and the used mixed acid containing the metal ions is discharged as a waste liquor.
The waste liquor, i.e. the mixed acid waste liquor containing the metal ions, is currently treated by a lime neutralization process and is discharged in public waters in a harmless form. However, the above treatment involves problems including formation of sludges of metal hydroxides and CaF.sub.2 in large amounts and an environmental problem that all the nitrogen contained in the nitric acid is discharged to public waters and causes eutrophication. In addition, the discharge of the expensive, unreacted acid components presents a problem from the standpoint of resource saving.
To solve the problems, many studies have been made on the processes for the regeneration and recovery of nitric and hydrofluoric acids from the waste liquor, including an extraction and distillation process using sulfuric acid, a solvent extraction process and a diffusion dialysis process. Some of these processes are practically in use with problems to be solved from the standpoints of economy, recovery rate and environmental protection.
Accordingly, further studies have been recently made. For instance, there has been proposed in published Japanese Patent Application No. 62-502695 a process of regenerating a nitric acid and hydrofluoric acid waste liquor by the use of a bipolar membrane (a membrane composed of anion and cation exchange layers). In this process, the mixed acid waste liquor containing metal ions is neutralized to cause the metal ions to precipitate and is separated into the precipitate and an aqueous solution of salts, thereby removing the metal ions. The aqueous salt solution obtained after the neutralization and separation is subjected to an ion-exchange electrodialyzer using a combination of a bipolar membrane, an anion-exchange membrane and a cation-exchange membrane, thereby regenerating and recovering nitric acid and hydrofluoric acid and alkalis.
The above process can be advantageously carried out by a closed system in which any waste liquor is not discharged from the treating system. Accordingly, the waste liquor can be regenerated without causing any environmental problem and discharging expensive unreacted acid components. Thus, the above mixed acid waste liquor regeneration process is beneficial from the viewpoint of environmental protection and resource saving.
However, the mixed acid regeneration process using the bipolar membrane is disadvantageous in that when a mixed acid water liquor having a high content of nitric acid is treated, a current efficiency becomes very low in the step of regenerating and recovering nitric and hydrofluoric acids and alkalis by means of an ion-exchange electrodialyzer, making the regeneration and recovery almost impossible in an industrial sense.